Method for monitoring the air bleeding quality of an electrohydraulic braking system

ABSTRACT

The invention discloses a method for determining the air content of the brake fluid in a hydraulic braking system. This method also permits determining the quality of the bleeding of the braking system. The principle of the method is a comparison of the volume intake with the respectively achieved pressure. The correlation is compared to nominal values. As the volume intake is determined by the clearance, the system is set by means of a pre-actuation to an initial condition before each actual measurement.

TECHNICAL FIELD

The present invention generally relates to a method for testing thebleeding quality and more particularly relates to a method for testingthe bleeding quality of an electrohydraulic braking system, comprising apedal-operated master brake cylinder and a brake circuit controlled bythe master brake cylinder pressure and including a pump.

BACKGROUND OF THE INVENTION

A braking system of this type is filled with an incompressible brakefluid. The dimensioning of the individual components (master brakecylinder, high-pressure accumulator, wheel brakes, etc.) is adapted tothe elasticities and clearances in the system. It does not take intoaccount that the brake fluid itself, when enriched with air or watervapor, becomes compressible and contributes to the elasticity of thesystem. It is therefore necessary in a conventional system to exchangethe brake fluid on a regular basis. Admittedly, this is also required inan electrohydraulic braking system, however, the system acts tolerantlywith respect to an enrichment of the brake fluid with air because thepump and the high-pressure accumulator can supply an adequate rate ofpressure fluid to be able to compensate an increased volume intake thatis due to air inclusions. If, however, the electrohydraulic brakingsystem switches to the conventional mode because e.g. the pump or thehigh-pressure accumulator has failed, the compressibility of the brakefluid will have a negative effect. As the pedal travel is limited it isonly possible to reach a defined braking pressure which, depending on towhat extent the brake fluid is enriched with air or water vapor, will nolonger be adequate to achieve the deceleration of a vehicle mandated bylaw. An extension of the pedal travel allowing the driver to senseenrichment with gas in the conventional system is not imperativelyavailable in the electrohydraulic system because the wheel brake isseparated from the pedal area by operation of the separating valves.

BRIEF SUMMARY OF THE INVENTION

In view of the above, electrohydraulic braking systems providemonitoring modes being employed during operation and trying to concludeany air enrichment from the behavior of the system. These monitoringmodes suffer from the disadvantage that some parameters which arerequired for the determination of the air enrichment cannot be definedwith a sufficient rate of precision so that conclusions made from thedata obtained with respect to the condition of the system are possibleto a limited extent only. Thus, e.g. the clearance of the brake pads inrelation to the brake discs is dynamically changed on account of therespective driving situation and, thus, undefined. It is alsosignificantly different from the static clearance, which adjusts itselfafter a brake application so as to be reproducible within relativelynarrow limits, with the vehicle at standstill.

Further, it cannot always be prevented that air is enclosed in thebraking system after brake fluid exchange has been carried outincorrectly. The inclusion of air should preferably be detected after abrake fluid exchange before starting to drive still.

Thus, the invention is based on the problem of developing a method fortesting the bleeding quality of an electrohydraulic brake system,wherein the air or gas load in the brake fluid or the inclusion of airbubbles can be reliably detected. To this effect, the method providesthe following steps:

-   a) Bleeding the high-pressure accumulator.-   b) Re-filling the high-pressure accumulator, during which action the    filling degree of the high-pressure accumulator and the volume flow    through the pump is determined.-   c) Comparing the actual values determined in this process with    preset specifications.-   d) Applying the brake pedal when the separating valves are closed,    so that a defined amount of pressure is applied to the wheel brakes    connected to the brake circuit.-   e) Releasing the brake pedal.-   f) Re-applying the brake pedal, with at least the following data    being detected:    -   pedal travel,    -   master brake cylinder pressure,    -   wheel brake cylinder pressure,    -   pressure fluid removal from the high-pressure accumulator.-   g) Determining the volume intake of the master brake cylinder and    the conduit reaching up to the separating valve by way of the    determined pedal travel and the master brake cylinder pressure and    comparison with the nominal values.-   h) Determining the volume intake of the pressurized brake circuit by    way of the pressure fluid removal from the high-pressure accumulator    and the wheel brake cylinder pressures and comparison with the    nominal values.-   i) Delivering an appropriate warning when predetermined criteria are    not satisfied.

In principle, pressure is applied to the system in a defined manner, andthe volume intake is measured as this occurs. The pressure is adjustedonly by pressure build-up, without pressure reduction. It is, however,necessary to this end that the braking system is moved into a normalcondition beforehand so that it can be supposed for the followingcalculations that defined parameters prevail within narrow limits. Forthis purpose, initially the high-pressure accumulator is bled andsubsequently re-filled, while the filling degree of the high-pressureaccumulator and the volume flow through the pump is determined. Thisway, a defined filling degree of the high-pressure accumulator is firstof all obtained. On the other hand, actual values in terms of the volumeflow and the filling degree of the high-pressure accumulator can becompared with preset specifications.

When deviations appear, it can be inferred therefrom that there was, andpossibly still is, an air inclusion at the suction side of the pump,meaning in the area of conduits between the pump and the pressure fluidreservoir, whereupon a corresponding warning can be delivered.

Likewise the application of the brake pedal according to step d) and thesubsequent release of the brake pedal brings about that in particularthe clearance of the wheel brakes and the position of the sealingsleeves in the master brake cylinder assume a quantity which can beestimated in good approximation.

Re-application of the brake pedal, while simultaneously a controlledbrake application commences, is used for measuring pressure and volumeintake, for what purpose according to partial step f) the pedal travelof the master brake cylinder pressure, the wheel brake pressures and theremoval of pressure fluid from the high-pressure accumulator aredetermined. The volume intake of the master brake cylinder and theconduits reaching up to the separating valve can be determined from thecomparison of the pedal travel with the master brake cylinder pressure.A comparison with nominal values can take place also in this event. Thevolume intake of the pressurized brake circuit is determined by way ofthe removal of pressure fluid from the high-pressure accumulator and thewheel brake cylinder pressures which, in turn, are compared to nominalvalues. Deviations allow conclusions with respect to the air load or theinclusion of air bubbles. In order that the model calculations necessaryfor this purpose found on a reliable basis, the comparisons are carriedout only when a master brake cylinder pressure amounts to e.g. >15 barand a maximum pedal application speed of <10 mm/s was maintained. Thepressure build-up in the wheel brakes shall amount to >30 bar. In suchcases, the measured values can be determined with a sufficient rate ofaccuracy, and it can be presupposed that model assumptions in terms ofthe volume intake apply with a sufficient rate of accuracy.

Further, the method shall only be performed when the temperature of thebrake system is in a predefined normal range and amounts to +10° C. to+30° C.

To be able to reconstruct the results, the results are transmitted to areadable electronic memory.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is the hydraulic wiring diagram of a hydraulic braking system.

FIG. 2 is a flow chart showing the method of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First of all, reference is made to FIG. 1. FIG. 1 shows a typicalelectrohydraulic braking system of the following design:

A tandem-type master brake cylinder comprises two brake circuits, one ofwhich is illustrated, and the illustrated primary brake circuit 2 isconnected to a pedal travel simulator 3. Further, a pressure fluidreservoir 4 is connected to the master brake cylinder 1. Pumps 5 drivenby a motor and a high-pressure accumulator, e.g. a metal pleated-bellowsaccumulator 6, form a pressure fluid supply system being supplied withpressure fluid (brake fluid) out of the pressure fluid reservoir 4. Thewheel brakes 7 of the rear axle are connected to this pressure fluidsupply system by way of each one inlet valve 8 and, as a result, providea controlled brake circuit 2′. In addition, a connection to the pressurefluid reservoir 4 can be established by way of one outlet valve 9 each.The inlet and the outlet valves 8, 9 are normally closed. Pressureincrease in the wheel brakes 7 occurs by opening the respective inletvalve 8, while pressure decrease occurs by opening the respective outletvalve 9. A controlled brake circuit 2′ is provided this way, and thepressure delivered to the wheel brakes 7 is determined by the masterbrake cylinder pressure, while the master brake cylinder 1 ishydraulically separated from the wheel brakes 7 in the event of acontrolled braking operation. To this end, a separating valve 10 isincorporated in the conduit that opens into the conduits leading to thewheel brakes 7 downstream of the inlet valve 8, said separating valvebeing closed in the control mode and staying open only when thecontrolled brake circuit 2′ is disturbed, e.g. due to failure of thepressure fluid supply.

The system is monitored and controlled among others by way of variouspressure sensors. Braking pressure sensors 11 are associated with theindividual wheel brakes 7, while a pump pressure sensor 12 is associatedwith the pressure fluid supply system, and one simulation pressuresensor 13 per brake circuit is associated with the master brakecylinder. The separating valve is closed during an electrohydraulicbraking operation. The pressure in the master brake cylinder is used asa control quantity. To this end, the pressure in the master brakecylinder 1 is measured by means of the simulation pressure sensor 13 andsent as a control quantity to the control of the controlled brakecircuit 2′. The separating valve 10 remains open upon failure of thepressure supply system. Thus, the wheel brakes 7 are connected directlyto the master brake cylinder 1 in a conventional fashion. As thisoccurs, a compensating valve 14 ensures a pressure balance between thewheel brakes of an axle.

Maintenance of the brake system is necessary in regular intervals,especially exchange of the brake fluid is needed. Errors may occur inthis respect. For example, it is likely that connections areinterchanged or that the brake system is not bled properly, what causesair inclusions. Therefore, the following process is suggested forchecking the braking system:

The process is menu-driven, its run being represented in FIG. 2. Thismeans an onboard computer with a display guides the driver or the fitterin executing the individual steps. The process can be carried out‘voluntarily’, e.g. upon the driver's request, by invoking acorresponding menu. The process can, however, also be carried outforcibly in that corresponding instructions are given to the driver(fitter) in a menu-controlled manner, and said instructions have to beprocessed before the vehicle can be set working. In both cases theprocess starts with the request (step 20) sent to the driver to fullyrelease the brake pedal of the master brake cylinder 1, what can bedetected by means of the sensor equipment. Another precondition fortriggering the process is that the vehicle is at standstill.

Subsequently, the memory condition is found out by means of a travelsensor 15 at the metal pleated-bellows accumulator 6 (step 21). Unlessthe accumulator is completely bled except for a defined residualquantity so that a bottom valve 16 actuated by the pleated bellows isclosed, the inlet and outlet valves 8, 9 are opened so that the pressurefluid flows out of the metal pleated-bellows accumulator 6 into thepressure fluid reservoir 4. Also, the fluid level in the pressure fluidreservoir 4 (step 22) is checked.

Because the brake system is completely unpressurized in this condition,it is also necessary that all pressure sensors 11, 12, 13 in the systemregister this condition. This fact can be used to adjust these pressuresensors, with at least the inlet valves 8 being opened, so that thepoints monitored by the pressure sensors are hydraulically connectedand, thus, assume an equal pressure level. Thereafter the metalpleated-bellows accumulator 6 is charged (step 23), and then pressure ofat least 30 bar is built up in the wheel brakes 7 (step 24) with theseparating valve 10 closed, the inlet valves 8 opened, and the outletvalves 11 closed. In case pressure buildup in the wheel brakes 7 doesnot occur in spite of open inlet valves 8, this is an indication thatthe connections of conduits at a valve block accommodating the valves 8,9, 10, 14 have been interchanged. Subsequently, the pressure in thewheel brakes 7 is decreased again by opening the outlet valves 9. Theclearance of the wheel brakes 7 is thereby given a defined quantity.

Thereafter the driver receives the request to apply the pedal slowly,evenly and powerfully (step 25). Separating valve 10 is closed duringthe actuation. Pressure increase is monitored (step 26), that means amaster brake cylinder pressure of at least 15 bar approximately shouldbe reached, and the speed of the pedal depressions should not exceed 10mm/s. In particular the master brake cylinder 1 and, thus, the positionof its sleeves is set into a normal condition with this initial brakingoperation. The pedal is then released again. The brake system now adoptsa basic condition with known parameters. If the mentioned values are notmaintained or reached, a new request will be issued.

Thereafter follows a second request to apply the pedal (step 27).Likewise this application is monitored in terms of the actuating speedand the pressure build-up achieved. As this occurs, the separating valve10 is also closed. A controlled pressure build-up takes placesimultaneously in the wheel brakes (step 28), and pressure of at least30 bar is reached. During this actuation of the master cylinder 1 andthe controlled pressure build-up, the pedal travel is measured (step 29)by means of a corresponding pedal travel sensor, the master brakecylinder pressure is measured by the simulator pressure sensor 13, thewheel brake cylinder pressures are measured by the wheel pressuresensors 11, and the volume intake is measured as well. The volume intakeis determined by the change of the degree of filling of the metalpleated-bellows accumulator 6 and the rate of delivery of the pumps 5.The data obtained is combined with each other and compared with nominalvalues (step 30). This comparison allows reliably detecting whether thebrake fluid is loaded with air, or whether air inclusions prevail.

With each of the above-mentioned steps all relevant measured quantitiesare monitored and compared with nominal values. If discrepancies aredetected, either the steps are repeated or warnings generated, andinitiation of the vehicle is prevented when very grave errors prevail.

Error messages can be, for example: ‘unit is defective’, ‘conduits areinterchanged’, ‘unit is poorly bled’, or ‘pumping capacity isinsufficient’.

1-4. (canceled)
 5. Method for testing the bleeding quality of anelectrohydraulic braking system, comprising a pedal-operated masterbrake cylinder and a brake circuit controlled by the master brakecylinder pressure and including a pump whose suction side is connectedto a pressure fluid reservoir and a high-pressure accumulator, as wellas inlet and outlet valves for the wheel brakes connected to the brakecircuit, with the master brake cylinder being connected to the brakecircuit downstream of the inlet valves by way of a separating valve,comprising the following steps: a) Bleeding the high-pressureaccumulator. b) Re-filling the high-pressure accumulator, during whichaction the filling degree of the high-pressure accumulator and thevolume flow through the pump is determined. c) Comparing the actualvalues determined in this method with preset specifications. d) Applyingthe brake pedal when the separating valves are closed, so that a definedamount of pressure is applied to the wheel brakes connected to the brakecircuit. e) Releasing the brake pedal. f) Re-applying the brake pedal,with at least the following data being detected: pedal travel, masterbrake cylinder pressure, wheel brake cylinder pressure, pressure fluidremoval from the high-pressure accumulator. g) Determining the volumeintake of the master brake cylinder and the conduit reaching up to theseparating valve by way of the determined pedal travel and the masterbrake cylinder pressure and comparison with the nominal values. h)Determining the volume intake of the pressurized brake circuit by way ofthe pressure fluid removal from the high-pressure accumulator and thewheel brake cylinder pressures and comparison with the nominal values.i) Delivering an appropriate warning when predetermined criteria are notsatisfied.
 6. Method as claimed in claim 5, wherein the pedalapplication is monitored in terms of the actuating speed and the masterbrake cylinder pressure achieved, and the method is only continued whendefined limit values are maintained.
 7. Method as claimed in claim 5,wherein the temperature of the brake system is determined and the methodis performed only when said temperature is in a predetermined normalrange.
 8. Method as claimed in claim 5, wherein the result of thecomparison is stored in a readable electronic memory.